A typical scroll compressor is shown in FIGS. 1 and 2. FIG. 1 is a cross-section of a scroll compressor 10, which comprises a fixed scroll 12 and an orbiting scroll 14. The fixed scroll comprises a generally planar disc 16 from which a scroll wall 18 extends perpendicularly. The orbiting scroll comprises a generally planar disc 20 from which a scroll wall 22 extends perpendicularly. A motor 24 is provided for rotating shaft 26. Shaft 26 has an eccentric shaft portion 28 fixed to the orbiting scroll 14. The eccentric motion of shaft portion 28 causes an orbiting motion of the orbiting scroll wall 22 relative to the fixed scroll wall 18. This relative motion causes fluid to be pumped from an inlet 30 provided at an outer radial portion of the scroll wall arrangement to an outlet, or exhaust, 32 provided at a radially central portion of the scroll wall arrangement. Gas enters the compressor through a compressor inlet (not shown).
FIG. 2 is a cross-section of the scroll wall arrangement of the scroll compressor taken along line II-II in FIG. 1. A fluid flow path 34 is shown in FIG. 2 by the arrowed line and follows a generally spiral path from the inlet 30 to the outlet 32 of the scroll wall arrangement. Gas enters through inlet 30 at a first pressure, is compressed over the course of four revolutions or wraps and is exhausted from the pump through outlet 32 at a higher pressure. The number of wraps can be more or less than shown in FIG. 2 and is selected depending on the pumping requirements. The relative orbiting motion of the scroll walls causes a plurality of crescent shaped pockets to be formed between the walls and forced radially inwardly, gradually being compressed in size. As is known to the skilled person, the extent of these crescent shaped pockets is approximately 360 degrees and the extent of the walls trapping a crescent shaped pocket is known as a wrap.
A scroll compressor is useful in that it is a lubricant free pump. Thus, a scroll compressor can often be adopted in mass spectrometer systems. A mass spectrometer system may include a differentially pumped series of chambers in which a plurality of chambers are pumped to different pressures and have respective interconnections between the chambers. The first chamber may be kept at a relatively high pressure (e.g. 2 to 10 mbar), with the last chamber being kept at a relatively lower pressure (e.g. 10−5 mbar). Typically, the low pressure chamber or chambers are pumped by a turbomolecular pump and the relatively higher pressure chamber or chambers are pumped by a primary pump. A scroll compressor is a suitable type of primary pump. As is known in the art, a turbomolecular pump requires a backing pump so that gas exhausted from the turbomolecular pump at a pressure less than atmosphere is pumped by a backing pump and exhausted at atmosphere. Such a differentially pumped system can therefore require at least three pumps: a turbomolecular pump, a backing pump and a pump for the relatively higher pressure chamber.
There is a desire to provide an improved pumping solution for the above mentioned problem and to provide a more versatile scroll compressor for pumping applications generally.